Photographic silver halide systems with ultrahigh contrast are used, for example, in reprography for the production of screened images from halftone recording elements, for photo typesetting technology and for line transparencies and photomasks. The term "ultrahigh" as used herein means that the contrast is higher than can be expected, if it is assumed that each individual emulsion grain is exposed and developed independently of its neighbors. Such systems use, for example, effects in which the development of a grain initiates the development of a neighboring grain even if the latter was not exposed sufficiently to be developable by itself ("infectious development").
So-called litho systems have long been known. These consist of films, in which the greatest proportion of the silver halide is present as chloride, and accompanying developers, which are characterized by a relatively high pH value, a low sulfite content and the absence of superadditive-acting developer compounds. Accordingly, the light sensitivity of the films and their development speed are relatively limited and considerable expense is required to maintain the activity of the developer constant over a longer period of time.
These disadvantages have been mitigated recently in systems introduced commercially, in which systems, the development of the photographic element is conducted in the presence of certain hydrazine compounds. Research Disclosure 23510 (Nov. 1983) presents a summary of the extensive literature on this subject. According to this publication, most so-called activated hydrazine compounds are used. Such compounds can be described by the general formula CT--NR.sup.1 --NR.sup.2 --Ac, wherein CT is a tertiary carbon, mostly as the constituent of an aromatic ring system, such as phenyl; R.sup.1 and R.sup.2 are alkali cleavable radicals and Ac is an activating group. The hydrazine compounds are usually added to the light sensitive layer of the photographic element. Free hydrazine compounds, which fog adjacent grains, are alleged to be generated by the action of the alkaline developer solution interacting with oxidation products generated from the developer compound in the development of the silver halide grains. In practice, preferred hydrazine compounds are those in which the activating groups are bonded through carbonyl groups to the hydrazo nitrogen. If CT in the above formula is a tertiary carbon in an aromatic group, these compounds are designated as aryl hydrazides.
A disadvantage of the systems using hydrazine compounds is that the development must be conducted at relatively high pH values. Indeed, the relevant publications describe developer pH values in the range from about 9 to 12.5; however, values over 11.5 are used exclusively in practice, because, otherwise, satisfactory development speed is not achieved and image quality is inadequate. In addition, the developer solutions are not stable enough for problem-free processing. In particular, they are very sensitive to atmospheric oxygen, despite their high sulfite content. Likewise, unavoidable slight fluctuations of pH during operation change the development or processing characteristics so strongly that it is difficult to maintain uniform results over a longer period. Further problems are the severe corrosion of the development machines by the highly alkaline developer solutions and the disposal of the comparatively highly buffered, exhausted solutions.
European Patent Application 02 53 665-41 discloses photographic elements that contain hydrazine compounds in which the activating group is split off in the alkaline developer medium with the formation of a ring-shaped structure. These elements can be developed at pH 11 with satisfactory results. The above indicated disadvantages are indeed mitigated hereby; but there exists as before a need for further improvement. Moreover, the particular aryl hydrazides used therein can be prepared only by multistep syntheses or with poor yields.
Although the hydrazine compounds appear today technically superior in many respects in the litho systems, there still exists in particular the need to further accelerate the processing operation, the duration of which is determined decisively by the development time.
Therefore, it is desired to provide photographic silver halide elements with specific hydrazine compounds that can be developed at relatively low pH values comparatively rapidly to ultrahigh contrast. It is also desired to provide elements of this type in which the result of development depends only slightly on pH value. Furthermore, it is desired to provide new hydrazine compounds that are suitable for the preparation of such photographic silver halide elements and can be prepared at low cost with good yield.